Tunable laser

Abstract

A tunable laser has a multiple ring resonator comprising a plurality of ring resonators having respective ring-shaped waveguides and respective different optical path lengths, an input/output side optical waveguide coupled to the multiple ring resonator, an optical input/output device such as a laser diode coupled to the input/output side optical waveguide, a reflection side optical waveguide coupled to the multiple ring resonator, an optical reflector coupled to the reflection side optical waveguide for removing light at an unwanted wavelength and reflecting light at a required wavelength, and a wavelength varying mechanism for changing the resonant wavelength of the multiple ring resonator.

1. A tunable laser comprising:
 a multiple ring resonator comprising a plurality of ring resonators having respective ring-shaped waveguides and respective different optical path lengths, the ring-shaped waveguides having circumferential lengths different from each other, the ring-shaped waveguides being connected in series; 
 a first optical waveguide coupling a first ring resonator to said multiple ring resonator; 
 a single optical input/output device directly coupled to said first optical waveguide, said optical input/output device being a laser diode or a semiconductor optical amplifier adapted to provide a single optical signal to a last ring resonator of the multiple ring resonator; 
 a second optical waveguide coupled to said multiple ring resonator; 
 an optical reflector coupled to said second optical waveguide, said optical reflector removing light at an unwanted wavelength and reflecting light at a required wavelength; 
 at least one third optical waveguide coupling adjacent ring resonators; and 
 a wavelength varying mechanism for changing the resonant wavelength of said multiple ring resonator, 
 wherein a free spectral range (FSR) of each ring resonator is different from one another, and a resonant wavelength changes depending on the circumferential lengths of the ring resonators. 
 
     
     
       2. The tunable laser according to  claim 1 , further comprising a substrate, wherein said multiple ring resonator, said first optical waveguide, and said second optical waveguide are disposed on said substrate. 
     
     
       3. The tunable laser according to  claim 2 , wherein said optical reflector has a reflecting film. 
     
     
       4. The tunable laser according to  claim 1 , wherein said optical reflector comprises a dielectric multilayer film. 
     
     
       5. The tunable laser according to  claim 3 , wherein said reflecting film comprises a dielectric multilayer film disposed on an end face of said substrate. 
     
     
       6. The tunable laser according to  claim 1 , wherein said optical reflector reflects only light in a wavelength varying range of said wavelength varying mechanism. 
     
     
       7. The tunable laser according to  claim 1 , wherein said optical reflector reflects at least light having a wavelength in a wavelength varying range of said wavelength varying mechanism. 
     
     
       8. The tunable laser according to  claim 1 , wherein said optical reflector reflects light at wavelengths included in and longer than a wavelength varying range of said wavelength varying mechanism. 
     
     
       9. The tunable laser according to  claim 1 , wherein said optical reflector reflects light at wavelengths included in and shorter than a wavelength varying range of said wavelength varying mechanism. 
     
     
       10. The tunable laser according to  claim 1 , further comprising at least one mode filter for preventing light in high-order modes from being introduced into said multiple ring resonator and propagating light in a fundamental mode in said multiple ring resonator, said at least one mode filter being built in at least one of said first optical waveguide, said second optical waveguide or said third optical waveguide. 
     
     
       11. The tunable laser according to  claim 1 , further comprising:
 a photodetector for detecting light propagated through said multiple ring resonator; and 
 a control circuit for controlling said wavelength varying mechanism based on the light detected by said optical detector. 
 
     
     
       12. The tunable laser according to  claim 11 , wherein said wavelength varying mechanism comprises a heater for changing the temperature of at least a portion of said multiple ring resonator. 
     
     
       13. The tunable laser according to  claim 2 , wherein said substrate comprises a PLC substrate. 
     
     
       14. The tunable laser according to  claim 11 , wherein said optical detector comprises a semiconductor photodetector. 
     
     
       15. The tunable laser according to  claim 5 , further comprising:
 a photodetector for detecting light propagated through said multiple ring resonator; and 
 a control circuit for controlling said wavelength varying mechanism based on the light detected by said optical detector. 
 
     
     
       16. The tunable laser according to  claim 15 , wherein said wavelength varying mechanism comprises a heater for changing the temperature of at least a portion of said multiple ring resonator, said substrate comprises a PLC substrate, said photodetector comprises a semiconductor photodetector, and said control circuit comprises a circuit for controlling an amount of electric current flowing through said heater to minimize an amount of light detected by said semiconductor photodetector. 
     
     
       17. The tunable laser according to  claim 5 , wherein said optical reflector reflects only light in a wavelength varying range of said wavelength varying mechanism. 
     
     
       18. The tunable laser according to  claim 5 , wherein said optical reflector reflects at least light having a wavelength in a wavelength varying range of said wavelength varying mechanism. 
     
     
       19. The tunable laser according to  claim 5 , wherein said optical reflector reflects light at wavelengths included in and longer than a wavelength varying range of said wavelength varying mechanism. 
     
     
       20. The tunable laser according to  claim 5 , wherein said optical reflector reflects light at wavelengths included in and shorter than a wavelength varying range of said wavelength varying mechanism. 
     
     
       21. The tunable laser according to  claim 5 , further comprising at least one mode filter for preventing light in high-order modes from being introduced into said multiple ring resonator and propagating light in a fundamental mode in said multiple ring resonator, said at least one mode filter being built in at least one of said first optical waveguide, said second optical waveguide or said third optical waveguide. 
     
     
       22. The tunable laser according to  claim 1 , further comprising a directional coupler coupling each optical waveguide to its respective ring resonator. 
     
     
       23. The tunable laser according to  claim 1 , wherein the first optical waveguide, the second optical waveguide and the third optical waveguide are each a straight optical waveguide.